I have a 2 yr old gooseberry bush that has grown out of a small gap between a concrete path and a stone outhouse wall, opposite and two metres away from a well established ordinary (green fruit) gooseberry bush. (There are lots of backcurrant bushes nearby, too.) The first year, there were just two or three green fruits; this year, there is a bumber crop, but they have all turned reddish-black in colour. There is no sign of insect disturbance or unhealthy stems or leaves, and, cut open, the fruits seem healthy and uninfested. They even taste okay, if a little more tart than regular gooseberries. Are they a safe mutation, or are they diseased? (I'd upload pictures if I knew how!!)

Worcesterberries , I remember being told as a child, were a gooseberry and blackcurrant cross, my mum had a plant from Woolworths. I think they are now thought to be a selection of Ribes divaricum or something like that, and jostaberries some complicated cross between all three. I think the pictured plant is probably some sort of cross. Very gooseberry type leaves but blackcurrant like fruit.

I agree with all the above and suspect it is a natural cross, probably more towards the jostaberry (I have worcesterberry and they are much more like a gooseberry than a blackcurrant.) Perfectly safe and well worth digging it out and replanting somewhere more appropriate - do that after the leaves fall and it goes dormant. The fruit should really show its worth when cooked and a jam would likely be delicious!